An aircraft turbojet engine nacelle generally has a structure comprising an air intake upstream of the turbojet engine, a middle section intended to surround a fan of the turbojet engine, a downstream section housing thrust reverser means and intended to surround the gas generator of the turbojet engine, and generally ends with a jet nozzle whereof the outlet is situated downstream of the turbojet engine.
The air intake of an aircraft nacelle comprises, on the one hand, an air intake lip adapted to allow optimal collection towards the turbojet engine of the air needed to supply the fan and compressors inside the turbojet engine and, on the other hand, a downstream structure on which the lip is fastened and intended to suitably channel the air towards the vanes of the fan. The assembly is attached upstream of a case of the fan belonging to the upstream section of the nacelle.
The formation of ice on the leading edges of an aircraft wing or on the air intake lips of the turbojet engines poses a number of problems, including: the addition of weight, imbalances between the port and starboard parts and, in the specific case of turbojet engine air intakes, the formation of blocks of ice of a nature to penetrate the turbojet engine and cause considerable damage, or to impact the airframe of the aircraft such as the engine nacelle stub, the wing, the tail group or the fuselage. In particular, depending on the temperature and humidity conditions, ice can form on the nacelle at the outer surface of the air intake lip. The presence of ice or frost alters the aerodynamic properties of the air intake and disrupts the conveyance of the air towards the fan. Bits of ice may detach from the air intake lip and collide with components of the turbojet engine such as the vanes of the fan, or the aircraft airframe.
The performance of the turbojet engine being related to the quantity and quality of the air collection done by the air intake, the air intake lip should be de-iced when ice or frost forms thereon. To that end, a number of de-icing or anti-icing systems have been developed in the aeronautics field. As a reminder, de-icing comprises evacuating the ice already formed, and anti-icing comprises preventing any ice from forming.
Anti-icing is necessary in particular in the case of turbojet engines comprising parts made from composite materials, such as the fan vanes: in such a case, it is necessary to eliminate any risk of ice arriving in the engine, the composite materials risking substantial damage in case of impact.
In the rest of this description, the term “de-icing” is used indifferently to designate de-icing or anti-icing.
Among the de-icing systems of the prior art, electric systems are known. An array of electric resistances is powered using a current created by electrical power supply members of the aircraft. These resistances are generally arranged in the skin of the leading edge or the air intake lip. These electric systems are very exposed to impacts of all kinds and in the case of perforating damage, they become problematic, if not impossible, to repair.
It is also known from the art, in particular from patent EP 1 495 963, to apply a heating resistor on an outer wall of the air intake lip. The heating resistor is subject to many impacts that can cause premature wear thereof, or even a malfunction thereof.
Such a malfunction of the heating resistor can cause ice or frost to build up on the air intake lip and therefore a decrease in the turbojet engine's performance.
Furthermore, the de-icing systems of the state of the art can be made up of conductors arranged in bands powered by different power supplies to limit breakdown problems due to a power shut-off. However, although part of the device is still powered in the event of a periodic power failure, blocks of ice appear on the entire length of the non-powered bands.
Such blocks of ice are large and can cause substantial damage when they are removed from the wall.
This is even more true in that the risks of tearing off are frequent since as the ice is deposited, the zone becomes thermally isolated from the outside air flow by ice accumulation and the temperature of the wall of the lip increases through the heat conduction effect resulting from the adjacent zone that is still powered. The interest of minimizing the size of these accumulations is therefore understood.